After another four consecutive breaks of serve from midway in the decider, Collins snatched the crucial final game at 5-4 as Halep served to stay in the match.
Despite losing, it was an impressive showing from the former champion in Montreal considering it was her first match in three months, but she previously admitted that she is still far away from her top form.
Speaking ahead the match, Halep said: “I’m still far from my best level. I don’t expect big things from this tournament. I’ve spent the last three months at home and spent my time doing things that we tennis players aren’t used to doing.
“I just wanted to recover physically. I am very excited about my return and I hope to be able to win as many starts as possible. I really like the playing conditions here in Canada.
“I have many good memories and I have won many good games. Every time I come I feel at home. I like the court, I like the warmth of the spectators. It really is a special place for me.”